[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <20200706162633.GA13288@paulmck-ThinkPad-P72>
Date: Mon, 6 Jul 2020 09:26:33 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Marco Elver <elver@...gle.com>,
Nick Desaulniers <ndesaulniers@...gle.com>,
Sami Tolvanen <samitolvanen@...gle.com>,
Masahiro Yamada <masahiroy@...nel.org>,
Will Deacon <will@...nel.org>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Kees Cook <keescook@...omium.org>,
clang-built-linux <clang-built-linux@...glegroups.com>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>,
linux-arch <linux-arch@...r.kernel.org>,
Linux ARM <linux-arm-kernel@...ts.infradead.org>,
Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
LKML <linux-kernel@...r.kernel.org>, linux-pci@...r.kernel.org,
"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>
Subject: Re: [PATCH 00/22] add support for Clang LTO
On Fri, Jul 03, 2020 at 07:42:28AM -0700, Paul E. McKenney wrote:
> On Fri, Jul 03, 2020 at 03:13:30PM +0200, Peter Zijlstra wrote:
> > On Thu, Jul 02, 2020 at 10:59:48AM -0700, Paul E. McKenney wrote:
> > > On Thu, Jul 02, 2020 at 10:20:40AM +0200, Peter Zijlstra wrote:
> > > > On Wed, Jul 01, 2020 at 09:03:38AM -0700, Paul E. McKenney wrote:
[ . . . ]
> > Also, if C goes and specifies load dependencies, in any form, is then
> > not the corrolary that they need to specify control dependencies? How
> > else can they exclude the transformation.
>
> By requiring that any temporaries generated from variables that are
> marked _Dependent_ptr also be marked _Dependent_ptr. This is of course
> one divergence of _Dependent_ptr from the volatile keyword.
>
> > And of course, once we're there, can we get explicit support for control
> > dependencies too? :-) :-)
>
> Keep talking like this and I am going to make sure that you attend a
> standards committee meeting. If need be, by arranging for you to be
> physically dragged there. ;-)
>
> More seriously, for control dependencies, the variable that would need
> to be marked would be the program counter, which might require some
> additional syntax.
And perhaps more constructively, we do need to prioritize address and data
dependencies over control dependencies. For one thing, there are a lot
more address/data dependencies in existing code than there are control
dependencies, and (sadly, perhaps more importantly) there are a lot more
people who are convinced that address/data dependencies are important.
For another (admittedly more theoretical) thing, the OOTA scenarios
stemming from control dependencies are a lot less annoying than those
from address/data dependencies.
And address/data dependencies are as far as I know vulnerable to things
like conditional-move instructions that can cause problems for control
dependencies.
Nevertheless, yes, control dependencies also need attention.
Thanx, Paul
Powered by blists - more mailing lists
Confused about mailing lists and their use?
Read about mailing lists on Wikipedia
and check out these
guidelines on proper formatting of your messages.
